A substantial amount of interest has recently been centered on the development of ambient temperature, high energy density, electrochemical cells which are light in weight and capable of providing a higher voltage than conventional cells such as nickel-cadmium and lead-acid systems or alkaline cells having zinc anodes. The high energy density cell systems which are currently of interest typically involve the use of active metals (metals with reduction potentials which are more negative than that of hydrogen in the electromotive series of elements in an aqueous environment) as anodes in combination with nonaqueous electrolytes. As used herein, "nonaqueous" is intended to mean substantially free of water. Lithium has been of particular interest as an active metal for such high energy density cells since it is the most active of the metals in the electromotive series and has the ability in an electrochemical cell to provide the highest performance in watt-hours per kilogram of all known active metals.
In conventional electrochemical cells, cathode depolarizers are used in a form which will permit an external electrical circuit, such as a set of wires connecting the electrodes of a cell, while also effecting a physical separation of the cathode depolarizer from the anode. In such cells, the cathode depolarizer is generally an insoluble, finely divided solid which is either admixed with or used as a coating over an inert conducting material, such as a nickel or carbon rod, which serves as a current collector or cathode. The physical separation of the cathode depolarizer from the anode is necessary to prevent a direct chemical reaction between the anode material and the cathode depolarizer which would result in self-discharge of the cell.
Until recently, it was generally believed that a direct physical contact between the cathode depolarizer and the anode could not be permitted within an electrochemical cell. It has been discovered, however, that certain cathode depolarizers do not react chemically to any appreciable extent with active metal anodes at the interface between the anode and the cathode depolarizer. Accordingly, with materials of this type, it is possible to construct an electrochemical cell wherein an active metal anode is in direct contact with the cathode depolarizer. For example, U.S. Pat. No. 567,515 issued to Maricle et al. on Mar. 2, 1971, discloses the use of sulfur dioxide as a cathode depolarizer in such a cell.
British patent specification No. 2,124,821 is directed to an electrochemical cell which contains an active metal anode, a solid active cathode, and an electrolyte which is comprised of a liquid solvate-complex of sulfur dioxide and an alkali or alkaline earth metal salt. It is disclosed that lithium tetrachloroaluminate (LiAlCl.sub.4) is a suitable salt and that the solvate can be prepared by reaction of sulfur dioxide with a stoichiometric mixture of the Lewis acid and base components of the salt, AlCl.sub.3 and LiCl. It is further disclosed that an organic cosolvent, such as acetonitrile, dimethoxyethane and propylene carbonate, can be used as an electrolyte component in combination with electrolyte salts that are not normally soluble in sulfur dioxide alone. However, there is no suggestion of the use of aluminum chloride as an electrolyte component.
U.S. Pat. No. 3,493,433, issued to Hoffmann on Feb. 3, 1970, discloses a nonaqueous electrochemical cell which contains a lithium anode and a solution of lithium tetrachloroaluminate (LiAlCl.sub.4) in liquid sulfur dioxide as an electrolyte. In addition, the plating of lithium is described from a solution which is composed of propylene carbonate which is saturated with both LiAlCl.sub.4 and sulfur dioxide. Similarly, U.S. Pat. No. 3,953,234, issued to Hoffmann on Apr. 27, 1976, discloses an electrochemical cell which contains a lithium anode and an electrolyte which is composed of an electrolyte salt dissolved in a mixture of sulfur dioxide and at least one organic cosolvent having no acidic hydrogen atoms and containing an element having at least one unshared electron pair. However, neither of these two references contains any suggestion of the use of aluminum chloride as an electrolyte component.
U.S. Pat. No. 4,375,502, issued to Gabano on Mar. 1, 1983, is directed to a lithium-thionyl chloride electrochemical cell wherein the electrolyte is composed of a solution in thionyl chloride of at least one salt selected from the group consisting of lithium tetrachloroaluminate and lithium hexachloroantimonate and a complex selected from the group consisting of AlCl.sub.3.SO.sub.2 and SbCl.sub.5.SO.sub.2. It is also disclosed that a second solvent, such as phosphoryl chloride or sulfuryl chloride, can be added to the thionyl chloride. However, it is further disclosed that the electrolyte does not include an excess of sulfur dioxide over that required for complex formation with aluminum chloride or antimony pentachloride.
U.S. Pat. No. 3,508,966, issued to Elsenberg on Apr. 28, 1970, discloses a nonaqueous electrolyte for use in a nonaqueous electrochemical cell wherein the anode is selected from the group consisting of lithium, sodium, calcium and magnesium and the active cathode material is a halide of a metal selected from the group consisting of copper, silver, iron, nickel and cobalt. The electrolyte is a solution which is composed of: (a) an organic solvent, such as propylene carbonate, (b) a Lewis acid, such as aluminum chloride, in combination with at least an equimolar amount of a coordinating compound, such as lithium chloride, which will react with the Lewis acid to yield a complex, and (c) a saturator salt which is a metal halide salt having a cation of the element used as the anode metal and the same anion as that of the active cathode material. This reference contains no suggestion of the use of sulfur dioxide as an electrolyte component and fails to suggest the use of unreacted aluminum chloride as an electrolyte component.
German Offenlegungsschrift No. DE 3431134 Al, dated Mar. 6, 1986, discloses a primary electrochemical cell which contains a lithium electrode, an organic polymer which is electrochemically oxidizable and/or reduceable as the active material of a second electrode, and an electrolyte which is composed of a solution of MHal in sulfur dioxide where M can be Al, Fe or Ti and Hal can be Br or Cl. However, this Offenlegungsschrift contains no suggestion that a solution of aluminum chloride in sulfur dioxide could be used as an electrolyte in an electrochemical cell wherein sulfur dioxide is electrochemically reduced. Further, this reference contains no suggestion of the use of a polar organic compound as an electrolyte component.
Rechargeable lithium-sulfur dioxide electrochemical cells which employ conventional electrolytes are typically characterized by a low discharge capacity and also by poor cycling behavior with reversibility problems occurring at both anode and cathode. As a result of these limitations, the utility of such cells has been severely limited.